The Double Edged Sword: The Impact of the Interaction between Salt and Sediment from Sub-Salt Exploration Risk in Deep Water

Selim S. Shaker
G.A.S., E.Stroudsburg, PA

The high potential rewards of the sub salt plays in the deep water mini-basins and frontier fold belts make it very attractive for exploration endeavors. However, complex subsurface geopressure can cause hydrocarbon breaches in addition to recurrent drilling challenges that drastically increase operation costs.

Geopressure models, built based on several fields and dry holes data in the Tertiary-Quaternary, successfully justify the risk assessments in each case history. Salt’s unique petrophysical, i.e. density, impermeability and ductile nature, contribute to substantial changes in the pore pressure gradients, stress paths, sealing and hydrocarbon retention capacities in the host sediments.

The prolific fields are attributed to:
1- Salt enhances the retention capacity in the sub-salt, thrusted fault system in the fold belt and the sealing capacity in supra-salt.
2- Drilling does not require high mud density to reach objective targets below salt.
3- High production flow rate of the Plio-Pleistocene, particularly above the salt where pressure gradient is high.

The challenges which can lead to exploration failure:
1- The moderate to weak pressure gradient and sealing capacity below the salt can be a substantially cause of seal failure and weak water drive in the production phase.
2- Several casing points needed to successfully drill sediments above the salt due high pressure gradient
3- Drilling hurdles are common at the salt-sediment interface, especially at the base of the salt.

While a great deal is known about salt body delineation from geological and geophysical data, this article addresses salt related risk assessment from a geopressure standpoint.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas